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The VLA has recently been upgraded with new receivers for the 21-cm wavelength band which have vastly improved performance compared to the preceeding system. These allow synthesis of the HI line in the fields of clusters of galaxies to reach to much higher sensitivity than has been possible in earlier attempts. Synthesis observations provide complete coverage of the volume of a cluster, we obtain a radio selected catalog of galaxies which is an inventory of all the HI gas in the cluster. The synthesis technique has a multiplex advantage over other kinds of observations : in one field we cover hundreds of galaxies, obtaining dozens of rotation curves, plus a map of the large scale variations of the systemic velocities over the cluster area.
This project is a demonstration of the power of this technique on the well known Hercules Cluster of galaxies (A2147 and A2151) at z=0.03 using the C and D arrays of the VLA to cover four primary beam areas (about one square degree). The resulting cube has resolution 18" and 41 km/s and detection threshold 1.6 x 10**8 h**-2 Msun. The richest field shows more than 50 galaxies detected in more than one channel each, plus many tentative lines. Some of these are enigmatic objects, such as galaxies with high HI mass but very low optical luminosity, which did not appear on previous, optically selected catalogs.
The most striking result of this study is a pronounced variation of the ratio of HI mass to optical light on a large scale from north to south across the cluster region. This appears to be the evolutionary process in action in this cluster; the removal of the gas follows the variation of morphological types in the population of galaxies. Following this same trend is a variation in the dynamical properties of the galaxies. Hercules is apparently a cluster in which we can trace, from one part to another, the changes of a population of galaxies during the growth of a rich cluster.
